This invention relates to thread cutting machines, and more particularly to a thread cutting machine in which the rotation of a thread cutting tool such as a screw tap is controlled according to the amount of movement of a spindle head which rotatably supports the thread cutting tool, thereby to cut the threads in a workpiece with high accuracy.
A tapping machine (or thread cutting machine) carries out a thread cutting operation with a screw tap (or thread cutting tool) initially abutted against a starting hole formed in a workpiece. Such devices include means for rotating a spindle to which the screw tap is secured and means for moving a spindle head, which rotatably supports the spindle, vertically towards the workpiece. The tapping machine, unlike a drilling machine, is to cut threads having a predetermined pitch in a starting hole formed in a workpiece, and therefore as the thread cutting operation with a screw tap is advanced, the screw tap is moved in accordance with the lead angle. Accordingly, it is necessary for the tapping machine that the spindle head is moved in the axial direction in synchronization with the amount of movement per revolution of the spindle holding the screw tap.
In order to meet this requirement, in the conventional thread cutting machine as disclosed, for instance, by Japanese Patent Application Publication No. 31356/1972, a gear is replaceably interposed in a drive system adapted to move the spindle head in the axial direction. In replacing a screw tap for cutting threads of different pitch, the gear is also replaced, so that the spindle head is moved in the axial direction with a lead angle corresponding to the new screw tap. However, the gear replacement system is disadvantageous because whenever the screw tap is replaced before a thread cutting operation, the gear must also be replaced by a gear suitable for the new screw tap. As a result, it is necessary to provide a number of gears according to the different screw taps.